But where do those numbers come from? And how accurate are they? “The algorithms these companies use to derive calorie counts is a black box—they’re not sharing,” says Cheung, “but it’s reasonable to assume those calculations are based on a combination of heart rate, weight, age, and maximum heart rate along with activity level.”

Depending on the information you input and the equipment you use, your calorie counts at the end of any given ride could be pretty spot on, in the general ballpark, or fairly far afield. Here’s what you need to know to make them as accurate as possible.

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Be Honest About Your Weight

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Honesty is the best policy, right? Well, if you want an accurate calorie reading from your bike computer it sure is. Why? Because the energy it takes for a 200-pound man to do a four-hour ride is radically different from the energy output of a 120-pound woman doing the same ride. Your age, gender, and body size heavily influence calorie calculations—especially if those are the only metrics your algorithm is working with (aka, you don’t use a power meter or heart rate monitor).

“Don’t say you weigh 60kg when you weigh 70kg. Don’t lie about your age. If you know your maximum heart rate, plug that in,” says Cheung. “When you’re setting up your profile, give the best data possible. The more accurate the numbers you put in, the more accurate the numbers you will get out.” That also means updating your profile if you gain or lose a significant amount of weight.

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Wear a Heart Rate Monitor

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The harder your effort, the higher your heart rate, the more energy you use. By wearing a heart rate monitor that is synced to your bike computer, you make your calorie counts more accurate than if those calculations are based solely on your body weight and riding conditions. By using a heart rate monitor, you also can perform a maximum heart-rate field test and input that number into your profile for an even more accurate calorie estimate, says Cheung.

For the most accurate calorie counts on your rides, use a power meter, which measures the work you are performing. You’ll get a number in kilojoules (kJ), a unit of work that recognizes the human body isn’t 100-percent efficient when turning energy from your fuel stores into physical work, like pedaling a bike. Power meters come in hubs, crank arms, pedals (seen here), and more.

“Conveniently, kilojoules and calories burned in cycling convert to a 1:1 ratio,” says Daniel Matheny, a USA Cycling level 1 elite coach and the head coach at Matheny Endurance in Colorado Springs. So, if you performed 500kJ of work, you can feel fairly confident, within a 5-percent margin of error, that you burned 500 calories, says Matheny, who will sometimes factors in kJs when assigning workouts to clients.

“When prepping an athlete for a specific event, like a multi-day stage race, I can make a pretty good estimate of what the week’s demands will be so I can ramp up workload based on kilojoules and I can prescribe that as a goal in their training cycle," he says.

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Keep Your Calories in Perspective

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Even if you have the most accurate calorie estimates possible, it’s important to keep those numbers in context, says Matheny, especially when you’re performing demanding workouts that don’t net high total kilojoules.

“If you do a VO2-max or sprint-interval workout where the efforts are really hard and short with subsequent really easy recovery intervals, you won’t end up with a huge kJ/workload expenditure, but you’re getting a big training effect,” he says.

So don’t automatically think, “I only burned 300 calories, so I won’t eat anything afterward.” Your body will require energy to recover. Likewise, if you’re out there for a few hours burning hundreds or even thousands of calories and eating along the way, you need factor in what you've consumed as well as what’s been burned off.

“Don’t look at the calorie-burned number at the end of the ride and immediately equate it with the amount of food you should or shouldn’t be eating afterward,” says Cheung. If weight loss is a goal, use those numbers as a ballpark to guide you, but keep them in perspective.

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